A radar system is described in German Patent Application No. 196 32 889, in which, however, the range finding is limited to the measurement of distances to solid bodies.
In contrast to this, the radar system according to the present invention has the advantage of being capable of creating distance profiles to diffuse objects, in particular fog banks. Thus, for example, quantitative information concerning visual range can be obtained in a motor vehicle independently of the driver. This functionality can be integrated here advantageously via simple additional technical measures in a device which is already used for distance sensing, for example for parking or for automatic vehicle control systems. Also advantageous is the simple feasibility of digital signal processing as a result of the relatively low-frequency difference signals to be used, for which inexpensive digital signal processors are available. Particularly advantageous is the provision of automatically operating compensation means, which permit realistic imaging of diffuse objects, in particular of fog banks. In particular, the driver can detect the actual extent and the actual spatial density distribution of such areas and thus realistically evaluate the traffic situation/obstacle to visibility.
If light which is frequency-modulated in its intensity is used as a transmitted signal, a real image of areas of possible obstacles to visibility is provided by the type of the measurement itself, since the transmitted signal is of the same physical nature as the light beams which are detected by the human eye and produce the optical image in the eye. Moreover, in the case of using light as an information carrier, as opposed to measurements in the radio frequency range, it is not necessary to pay attention to spectral purity or to a band limitation, so that frequency modulation taking account of harmonics can also be performed in a rectangular form without impairing the measurement result. In the case of systems in the radio frequency range, on the other hand, strict adherence to the assigned frequency bands (for the most part, moreover, very narrow-band ranges, cf. Industrial Scientific Medical bands) is necessary (key words: wireless service, electromagnetic compatibility, etc.); therefore a radar signal used in the radio frequency range cannot be simply a rectangular sequence, since the harmonic waves transmitted in this case would cause a xe2x80x9ccontaminationxe2x80x9d far from the assigned band. In addition, high-performance laser diode drivers, which are offered for optical communication and are designed for pulse modulation, may be used for the process of visual range determination.
Furthermore, the transmitted signal is emitted divergently in an advantageous way, in order to obtain a good averaging over the density of any existing fog layers.